An electrochemical sensor will be used to continuously monitor urea during hemodialysis. The sensor can be operated on the blood side or on the solution side. Detection is on a plated metal electrode and is a direct measure of the quantity of urea present. A membrane is provided to protect the sensor from blood. Additional measures are required to limit the response to urea only. PROPOSED COMMERCIAL APPLICATION The device will be used to permit direct monitoring of urea levels in the blood, the dialysis fluid effluent, or both during hemodialysis. There are over 200,000 dialysis patients in the United States receiving multiple treatments per week. Even limited penetration by this device represents a large market.